class Solution {
    public int climbStairs(int n) {
        //1 // 1 2 3 5 8 13

        //ret1 前面的数 ret2 后面的数
        int ret1 = 1,ret2 = 1;
        for(int i = 1;i <= n;i++) {
            int ret = 0;
            ret = ret2;
            ret2 = ret1+ret2;
            ret1 = ret;
        }
        return ret1;
    }
}